1. Field of the Invention
One of the problems associated with holograms is that diffraction optics displays utilizing a hologram are degraded by the effects of the spurious reflection and transmission hologram recordings that are generated during the holographic exposure process. These spurious holograms have been found to be generated by reflections from surfaces which are interfaces of materials of differing index of refraction, such as air to glass interfaces, of the surfaces of the recording cover plates, the substrate or the optical elements which interfere with the main holographic beams and with each other to form both spurious reflection hologram recordings and spurious transmission hologram recordings on the recording film. In a display utilizing a hologram the spurious reflection hologram recordings create ghost images and the spurious transmission hologram recordings create rainbow-like flare patterns. The purpose of this invention is to prevent the recording of the unwanted spurious holograms that are caused by reflections and thus to eliminate both ghost and flare images in the final display.
2. Description of the Prior Art
Prior to this invention there was no known method to reduce the flare and ghost images of holograms to acceptable levels. The basic approaches that have been utilized in an attempt to minimize the problem are either to anti-reflection coat all surfaces, to embed all optical elements in an index matching fluid when forming the holograms such that no air-to-glass interface reflections reach the active area, or to form the holograms with the beams impinging the recording medium at the Brewster's angle. The first method has been found to be inadequate because the best known anti-reflection coating provides results that are many times worse than required. The second method is generally unsuccessful because the striations in large fluid volumes degrade the holographic recording process. The third method is unsatisfactory because of the restriction of the angular placement of the recording, and hence playback, beams to the recording medium.